Stack exhaust design revisited: the 1 in 6 taper myth?

[JJG Koopmans]

(@Law)As far as the Adams vortex blastpipe is concerned, it consists of two blastpipes of which the orifices are flattened into a bananashape. As such it allows flow between the blastpipes. It was generally considered successful, however, being a casting it could not be repaired from wear as welding was not yet possible.

(@Soot)For the tall stacks, you should realize that the momentum from the jet is transferred to the combustion products. If a uniform velocity is assumed at the stack end and its diameter is 3 times that of the orifice its is easy to understand that 3 times the steam mass at 1/3 of its velocity is expelled as theoretical maximum if no vacuum is assumed. So taper increases the amount because of the wider exit and its influence on the chimney wall friction. A tall chimney is ok because it allows the momentum transfer over a longer path. The same effect can also be achieved with a shorter chimney and multiple orifices.
Kind regards
Jos Koopmans

[BryceGTX]

A few years ago when I was designing my cylinders and exhaust pipe, I specifically looked at the size of the pipe as I wondered why the American designers used a plenum in the exhaust passage.

It seems that the design of the engine requires the steam ways to have an area of 1/10 the piston area (at a particular engine speed). Then the blast nozzle ends up being about 1/20th the area of the piston. These are design rules for American locomotive engines. Turns out the blast nozzle on my 1/8th scale was also 1/2 the area of the steam way.

If we connect the larger steam ways and valve through a fixed size pipe (top picture), we see that the backpressure of the engine is determined by the blast nozzle!! This is clearly not what we want.

American locomotive designers solve the problem by inserting a plenum equal to about 1/2 the volume of the swept volume of the cylinder into the steam passage (bottom picture). This decouples the blast nozzle size from the exhaust of the cylinder. So my design also has a plenum in the exhaust pipe.

Any thoughts?
Bryce

A few years ago, I made this observation, but I was unable to provide specific design documents to show this. I was recently reading "Railway Locomotive Engineering" January 1898 and I found this discussion in "Volume 11" page 32. It supports my discussion on this other thread:

http://www.chaski.org/homemachinist/vie ... 6&start=24

Here is the text from "Railway Locomotive Engineering"

"After months of hysterical screaming that the running reported as having been
made by the Atlantic Railroad fast train during last summer was absurd, and
engineering impossibilities, the London Engineer has at last been forced to
admit that the "impossible" speed was accomplished. The painful admission is
now made that an American express locomotive can run faster than an English express
engine of similar proportions, and the whole editorial staff and correspondents
of the paper named are racking the inside of their skulls to find out the cause
of the difference."

"The only theory thus far advanced is a very lame one, and is to
the effect that owing to peculiarities and rigidity of structure, the English
locomotive progresses with a sinuous motion, the wheel flanges rubbing first on one line of
rails and then on the other, a large proportion of the power developed being
wasted in overcoming the flange resistance."

"As cut and worn flanges are less common on the British railway rolling stock than
they are on our cars and locomotives, the sinuous motion theory will not hold water.
We advise the men who are working on this stupendous problem to secure detail
drawings of an American express locomotive and compare the dimensions of all
the important parts."

"We have frequently heard the expression made use of by our ablest master me
chanics and steam engineers, that there is no difficulty in getting all the steam
necessary into the cylinders of a high speed locomotive, the difficulty is in get
ting it out quickly enough to prevent the pistons encountering the obstructive back
pressure. If the back pressure is excessive at the higher piston speeds the engine
will work against itself, so to speak, and a governor-like action will intervene
which limits the rotation velocity."

"We do not think, on the whole, that large steam ports and their corollary large
exhaust ports are conducive to economical use of steam, but there is no doubt
that large passages for letting the steam out of the cylinders help very much
in making an engine run freely. Our English friends argue that as American
locomotives have smaller nozzles than those used in Great Britain, the
obstruction to the free emission of the steam must be greater, but they
forget that the large volume of space between the exhaust opening and
the nozzle performs the part of a reservoir which holds the steam until
it can be pressed through the nozzle."

"Those who have listened to the exhaust of high speed British locomotives must have been struck
with the fact that the individual exhausts are much more distinct than they are with
an American locomotive working with the same piston speed. The cause of this,
no doubt, is that the exhaust pipe reservoir of an American engine tends to cause
a continuous exhaust, while the British engine, with its limited exhaust pipe capacity,
must jerk out the whole volume of exhaust steam the instant the exhaust
port opens."

"All the indicated diagrams which we have examined of British and American
locomotives taken at high speed corroborate this statement. While we think
that the features which make the American locomotive the fleetest in the world
lead to much waste of fuel, careful investigation will show that our immense
exhaust port is the only thing that causes the difference"

Perhaps others may find this useful..
Thanks,
Bryce

[JJG Koopmans]

1) using someone's 1898 opinion is not a strong argument in a scientific debate
2) just Googling "pulsating flow diffuser" will give a load of articles with conclusions like:
".....(2) The time-mean pressure rise is larger than that in the steady flow, ..."
just meaning that in a steam locomotive a pulsating exhaust increases the performance of the stack
kind regards
Jos Koopmans

[BryceGTX]

1) using someone's 1898 opinion is not a strong argument in a scientific debate
2) just Googling "pulsating flow diffuser" will give a load of articles with conclusions like:
".....(2) The time-mean pressure rise is larger than that in the steady flow, ..."
just meaning that in a steam locomotive a pulsating exhaust increases the performance of the stack
kind regards
Jos Koopmans

I don't see that research of any age is not relevant in this debate. Considering research prior to 1898 forms the basis of all research that follows.

But it is encouraging that you are finally warming to the obvious fact that we cannot consider any significant research in front end design without considering the effect of pulsation.
Bryce

[JJG Koopmans]

In my thesis of 2005 I devoted one and a half page to the effect of the Strouhal number which has exhaust beat frequency in it. At that time I noted already the research outside locomotives indicated increased performance with pulsation. My point of view is that in design and momentum calculations of a front end the effect can be neglected.
If I read the text and your comments correctly the plenum should act as a pulsation damper which imho decreases the performance as research on pulsation effects indicates otherwise.
kind regards
Jos Koopmans

[BryceGTX]

In my thesis of 2005 I devoted one and a half page to the effect of the Strouhal number which has exhaust beat frequency in it. At that time I noted already the research outside locomotives indicated increased performance with pulsation. My point of view is that in design and momentum calculations of a front end the effect can be neglected.
If I read the text and your comments correctly the plenum should act as a pulsation damper which imho decreases the performance as research on pulsation effects indicates otherwise.
kind regards
Jos Koopmans

If we consider only the simple affect on the blast, we miss the big picture..

The benefit to the large exhaust plenum comes from the increased power generated by the US engine due to low cylinder back pressure rather than the significant reduction in exhaust pulsations. The increase in back pressure of the British locomotives becomes exceedingly worse as the speed increases and particularly problematic at long cutoff and high speeds. This problem is clearly pointed out when the author says:

"If the back pressure is excessive at the higher piston speeds the engine will work against itself, so to speak, and a governor-like action will intervene which limits the rotation velocity."

It also seems clear that exhaust pulsations are a significant problem rather than a benefit. Any pulsation in the exhaust blast results in a disadvantage in the effective production of steam in the following ways:

1) Pulsations in the blast result in pulsations of air through the grate. These pulsations introduce blasts of cold air into the fire box and the lifting of coal bed producing large quantities of ash and sparks that are sent up the stack.

2) Pulsations in the fire tubes result in uneven heating between the gases and the fire tubes. Heat transfer is affected to a disadvantage.

3) Pulsations in the continuum of air from the entrance of the grate to the exhaust out the stack results in needless loss of energy due to the continual acceleration and deceleration of the large masses of air and gasses.

The biggest advantage of the exhaust pulsations is the "bark" that is so desirous to the steam locomotive aficionado. This in itself will be perhaps the overriding objection to the loss of exhaust pulses in small scale steam locomotives that we build.

On the flip side, US locomotives provide a solution to the defects shown above due to exhaust pulsations.

US steam locomotives of this era had large fire boxes to absorb the blast of air coming through the grate. The large fire box provided a huge volume for the air to mix with combustion products and complete the combustion process. A fire box arch significantly enhanced this process by routing the gases along a longer path assuring more complete combustion.

The large smoke box of US steam locomotives of this era provide a huge plenum where exhaust gases could accumulate until the next blast cycle. Baffles and screens within the smoke box enhanced this effect and helped to remove sparks. The baffles also provided a circuitous path to reduce uneven heating in the fire tubes assuring more complete transfer of heat to the water.

These fire box and smoke box design differences of US locomotives in addition to the exhaust passage plenum provided a decided advantage over the British designs of that era to the production of greater power at high speed. No doubt at the expense of efficiency.

This difference in design reflected the design requirements of the US market rather than any design defects of British locomotives. The British were much more concerned about efficiency at short cutoff. The US was much more concerned about power at long cutoff.

A significant point in this discussion is that the dynamic pulsing of the steam locomotive affects not just the blast, but rather the complete process of air to steam, to mechanical work. It's complete analysis requires a dynamic model as I have insisted earlier in this thread. Many of the design characteristics of US steam locomotives can only be understood in the dynamic sense.

Thanks,
Bryce